This invention relates generally to data signal transmission cables and methods and particularly to ones adapted for use in seismic prospecting. The invention relates even more particularly, but not by way of limitation, to a seismic telemetry method and wire cable with integral repeaters.
Seismic data can be useful to a person, such as a geologist, who analyzes subterranean formations in prospecting for oil and gas. Vibrations, or elastic waves, are generated in the earth by artificial means, such as explosives. Vibration detecting devices, called geophones, located in the area of interest pick up vibrations after they have passed through the subterranean formations. The geophones convert the detected vibrations to analog electrical signals which can be analyzed by known techniques to reveal characteristics of the subterranean formations.
Before the seismic data derived from the geophones can be analyzed, however, it must be converted to a form which can be understood by the analyst. Analog systems were first used to make this conversion. These converted the analog signals from the geophones into a strip chart graphical presentation, for example. More recently, digital systems have also been used. These digital systems typically have at least one data acquisition unit which receives analog signals from a number of geophones, converts them to digital format, and transmits the digital signals to a central processing unit. An example of a system utilizing data acquisition units and a central processing unit is the Halliburton Services' DFS-7 digital field system.
Speed is an advantage of the digital systems. Metallic wire cables have been used to connect data acquisition units with other data acquisition units and the central processing unit. Although the data acquisition units and the central processing unit can operate at higher speeds, transmission speeds over these metallic wire cables have been limited to about 4 megahertz because of the distances over which the data are typically transmitted. That is, the inherent impedance of the metallic wire cables significantly attenuates higher frequency signals over typical transmission distances. This inherent shortcoming of metallic cables has been overcome by fiber optic cables; however, the higher transmission speed advantage of fiber optic cables is at least partially offset by shortcomings of the fiber optic cables compared to metallic wire cables. These shortcomings include higher cost, lower trade acceptance, and physical deficiencies arising from the field environment where seismic prospecting is done.
There is, therefore, the need for a data signal transmission cable and method which are specifically applicable to seismic prospecting wherein digital equipment is used and which overcome the aforementioned shortcomings of prior metallic wire cables and fiber optic cables.